This invention relates to sewing machines in general and, in particular, to a new and useful stitch setting device for adjusting the length of stitch which is sewn by a sewing machine.
A stitch setting device is known from German Pat. No. 1,027,970, in which a manually rotatable cam disc is provided as the setting member, and has a recess which, beginning from a zero position, grows uniformly larger to both sides of a concentric zero line. The curved surfaces of the recess form stop faces for a contact pin which is secured to a manually pivotable support and is connected through a multi-member linkage to the feed elements of the sewing machine. A tension spring acting on the support holds the pin in contact with the outer curved face whose radial distance from the concentric zero line determines the forward feed step. To reverse the sewing direction, the support is pivoted and the pin is brought into contact with the inner curved face. Since in either of the two positions, the contact pin is radially equally spaced from the concentric zero line, the feed steps in the forward and backward directions are equal to each other.
In order to secure a seam, the threads are frequently locked on both ends of the seam by sewing a short length alternately forwardly and backwardly, two or three times. To ensure a continuous operation, the forward sewing must be reversed to the backward sewing and vice versa as quickly as possible. For this purpose, it is known to connect the support carrying the contact pin to an air cylinder and to determine the instant of reversal, for example, by means of photocell controls or stitch counters.
However, experience has shown that quick reversals of the stitch setting device cause a premature wearing down of the contact pin and the stop faces of the cam disc. In addition, the train members of the linkage between the air cylinder and the contact pin which transmit the setting forces from the air cylinder up to the respective stop face of the cam disc are exposed to high bending and buckling stresses caused by the abrupt braking during the motion reversal, and thus, may deform or even break.